Three forms of the IL2 receptor have been reported: low, intermediate and high affinity. Each form corresponds to the expression of two different receptor subunits: p55 a chain and p75 beta chain. Sole expression of p55 yields low affinity receptors, sole expression of p75 yields intermediate affinity receptors, and co-expression of both subunits yields high affinity receptors. Resonance energy transfer studies have suggested the presence of another peptide subunit of the IL2 receptor with an approximate molecular mass of 95,000 Daltons. The requirement of another protein to form a functional IL2 receptor is supported by experiments in which fibroblasts transfected with p75 CDNA express the protein on the surface but do no bind IL2, whereas transfected T cells express p75 and bind IL2. We have discovered and characterized the presence of a putative new IL2 receptor subunit with a molecular mass of 95,000 - 1 10,000 Daltons. This subunit is present in low, intermediate and, perhaps, in high affinity receptor complexes. Its presence is required to form intermediate affinity receptors with p75, but it is not necessary to obtain IL2 binding to p55. The p95 protein, termed the gamma subunit of the IL2 receptor, is not ICAM- 1 as determined by monoclonal antibody competitor studies.